1. Field of the Invention
The present invention relates to a PTC device which can be used as a circuit protection device and a battery pack or assembly using such a device.
2. Introduction to the Invention
The use, as a circuit protection device, of a PTC (positive temperature coefficient of resistance) device comprising a PTC composition which has the "positive temperature coefficient of resistance" property so as to protect batteries and/or circuits from overcurrent conditions and overtemperature conditions therefrom is disclosed in, for example, U.S. Pat. Nos. 4,255,698 (Simon) and 4,973,936 (Dimpault-Darcy et al), and Japanese Utility Model Application No. 4-75287.
The PTC composition is one which gives a device comprising the composition (i.e. a PTC device) the following characteristics: the device is in a low resistance condition at a temperature during a normal operation (depending on circuit conditions in which the device is used, for example from room temperature to 40.degree. C.); on exposure to a high temperature due to, for example, unusually large current resulting from the formation of a short circuit or excessive charging (depending on circuit conditions in which the device is used, for example from 60 to 130.degree. C.), the device switches into an extremely high resistance condition. Thus, when such a device is included in a circuit and an abnormal current passes through the circuit, the device comes into the higher temperature condition and switches into the higher resistance condition so that it decreases the current passing through the circuit and thus protects electric elements forming the circuit, parts connected to batteries and so on.
Such a PTC composition is well known and no additional description is required. Detailed description can be seen in, for example, Japanese Patent Kokai Publication No. 55-78406. The disclosures of these documents are incorporated herein by reference.
Battery packs, in which a plurality of batteries are contained, are commonly used in electric or electronic equipment such as cameras, video cameras, tools, portable phones and portable computers. It is desirable to make such equipment as compact (i.e. lightweight and small) as possible from a viewpoint of its portability. Thus, the battery packs contained in the equipment are preferably compact.
Abnormal current may pass through a circuit in such equipment due to the occurrence of a failure such as the formation of a short circuit. When a PTC device is used for the protection of the circuit from failure, effective space occupation of the device within the equipment is required from a viewpoint of the compact equipment.
In order for the equipment to be compact, U.S. Pat. No. 4,973,936 discloses placing the PTC device directly onto a button terminal of a battery. In this PTC device, a PTC component is in the shape of a disk to which a ring shaped insulation element is bonded using an adhesive, and a central portion of the insulation element is a hole which can receive the button terminal of the battery. For such a PTC device, since the PTC component is located on the top of the button terminal of the battery, a space is required in the battery pack to accommodate the thickness of the PTC component and to prevent the formation of a short circuit between the electrodes of the PTC component. Thus, the battery pack is required to have a sufficient space along a longitudinal direction of the battery.
Japanese Utility Model Application No. 4-75287 discloses a PTC device as a circuit protection device which comprises an electrode having a cap-shaped portion which can receive a button terminal of a battery, a half-ring-shaped electrode, and a PTC element sandwiched between the electrodes. With such a PTC device, since a distance between a side of the cap shaped portion of the electrode and the half-ring-shaped electrode is small, the formation of a short circuit is possible between those electrodes. In order to prevent such formation, a positional relationship between the cap portion and the PTC element and between the cap portion and the other electrode should be precisely adjusted. Such adjustment leads to an increase of production cost of the PTC device.